The present invention relates to a global navigation satellite system (GNSS) receiver, and more particularly, to a method of inter-channel bias (ICB) calibration in a GNSS receiver and related device.
Nowadays several global navigation satellite systems (GNSSs), such as Global Positioning System (GPS) and Global Orbiting Navigation Satellite System (GLONASS), have been developed and designed for commercial use. The navigation satellites of the same or different GNSSs may broadcast radio-frequency (RF) signals for positioning at different frequency bands. For example, the GPS navigation satellites broadcast RF signals at two frequency bands, around 1575.42 MHz (L1 frequency band) and 1227.6 MHz (L2 frequency band) respectively. In addition, the GLONASS navigation satellites broadcast RF signals at different frequency bands from each other. A GNSS receiver which supports multiple GNSSs (e.g. GPS and GLONASS) can receive RF signals from different GNSSs simultaneously. However, because the navigation satellites may broadcast RF signals at different frequencies, after the received RF signals are filtered, group delays relative to the received RF signals of different channels are different. Such different group delays induce frequency-dependent delays. Moreover, the GNSS receiver has an analog-to-digital converter for converting the received RF signals into digital baseband signals, and a digital processing circuit for generating measurement result. Such element and circuit also induce different hardware/software processing delays.
In general, the frequency-dependent delays and hardware/software processing delays can be separated into a common term and a delta term. The common term can be calibrated by a receiver clock bias of the GNSS receiver. The delta term, which is also called inter-channel biases, cannot be calibrated by such receiver clock bias and may degrade position accuracy of the GNSS receiver.
There are several approaches to enhance positioning accuracy for a known GNSS receiver. A first approach is to use GNSS based navigation information to offset group delays, as disclosed in U.S. Pat. No. 6,608,998 B1. However, the first approach needs to receive RF signals from one GNSS system (e.g. GPS/GLONASS) first for calibration. A second approach is to add an additional calibration circuit to generate a reference signal for calibration, as disclosed in U.S. Pat. No. 6,266,007. However, the second approach raises cost of the GNSS receiver. A third approach is to add an additional front-end calibration channel, where received RF signals are band-pass filtered at a signal intermediate frequency for avoiding group delays, so that group delays of other channels of GNSS systems can be calibrated accordingly. However, the third approach also makes cost of the GNSS receiver increased.